The present invention relates to radio communications systems and, more particularly, to low-cost mobile transceivers deployed in such systems.
During the past decade, there has been an enormous rise in the deployment of mobile telephony. After a relatively slow start with respect to radio systems based on analog technology, interest in mobile phones has escalated as digital technology has become available. Today, large parts of the world are covered by mobile networks which are available for commercial usage. Although analog systems such as AMPS, NMT and ETACS are still in operation and have been installed widely, most new subscribers adopt the newer digital systems such as GSM, D-AMPS, and PDC.
Since the mobile terminal has become a consumer product, there has been a constant drive toward cost-effective implementation. The general trend is to integrate as many functions as possible on a single integrated circuit (IC) or chip in order to reduce the number of external components required. Doing so not only reduces costs, but also reduces power consumption and increases reliability. Therefore, there is a constant search for radio architectures that allow this integration, and the ultimate goal is to produce a single-chip radio. In other words, a single integrated circuit on which all transceiver functions are provided.
A major obstacle with respect to on-chip implementation involves signal filtering. In particular, rigid requirements are placed on channel filters in order to suppress adjacent channel signals and in-band blocking signals. Most mobile receivers today use a superheterodyne-type structure in which the RF signal is subsequently down-converted and filtered in one or more IF stages. However, the required high-Q bandpass filters must have high selectivity and are difficult to integrate. Usually, off-chip filters like ceramic or crystal filters or Surface-Acoustic-Wave (SAW) filters are used for this purpose.
Another architecture which is typically more appropriate for on-chip integration is the homodyne receiver. Here, the down-conversion takes place directly to baseband or DC, in which case low-pass filters can be applied for selectivity which are easier to implement on chip. However, the homodyne receiver has problems as well. For example, there is typically DC off-set and second-order modulation which falls at DC and which cannot easily be distinguished from the information signal.
A third known radio architecture is the low-IF structure. In this architecture, the RF signal is down-converted to a low intermediate frequency which allows the usage of filters with a low Q. Such filters are easier to implement on chip. However, a problem associated with low-IF receivers relates to image signals. Multiplying an intended signal at a frequency f.sub.-- 1 with a local oscillator frequency f.sub.-- lo (f.sub.-- lo&gt;f.sub.-- 1) to provide an IF frequency at f.sub.-- lo-f.sub.-- 1 will also convert down any image signal arising at the other side of the local oscillator (LO) frequency (i.e., at f.sub.-- 2=2.times.f.sub.-- lo-f.sub.-- 1) down to the same IF frequency f.sub.-- lo-f.sub.-- 1. Therefore, rigid requirements are placed on image rejection since adjacent, in-band signals can be much stronger than the intended signal (note that in the heterodyne-type receiver the image band falls outside the communication band and is therefore suppressed by an RF filter which is present after the antenna and before the signal enters the receiver).
In addition to the push for on-chip integration, work is going on to increase the data rates in existing mobile systems. There are several possible techniques for obtaining higher data rates including multi carrier schemes. In a multi-carrier system, a communication link is assigned several carriers each supporting one channel. Thus, several low-rate channels are combined to form one high-rate link. However, multi-carrier systems tend to complicate transceiver design, and in general are not attractive if a single chip solution is desired.
Thus, there is a need for a radio architecture that can easily be implemented on chip, but which will also fulfill selectivity and image rejection requirements. There is also a need for an on-chip radio architecture that provides an increased data rate compared to current transceiver implementations.